1. Field of the Invention
The present invention relates to an image reading apparatus in which a plurality of light sources are divisionally lit on every block and irradiate an object to be read, thereby reading an image of this object.
The image reading apparatus according to the invention is widely used as an input unit of an image input apparatus such as, for example, facsimile apparatus or digital copying apparatus, or of other image processing apparatus.
2. Related Background Art
In recent years, the facsimile apparatus, the input apparatus of characters, images, and the like ordinarily use an image reading sensor which is constituted by arranging photosensors using thin film semiconductor, CCDs (charge coupled devices), or the like along the whole width of an original. As a light source, on the other hand, an LED array which is constituted by arranging a number of LEDs is used in order to reduce the size and weight.
A thin film semiconductor made of amorphous silicon, Cds:Se, or the like is frequently used as the image reading sensor. Such a semiconductor itself has good photoconductive characteristics. However, in the case of reading an original image by the monotonous color light of the LEDs or the like, it is difficult to perform the image reading operation at a high speed because the light response speed of the sensor itself is slow. Further, in the image reading apparatus using the LED array, a large current needs to flow through the LEDs in order to obtain a large amount of light enough to read the image. This causes the light amount to be reduced due to an increase in temperature of and the deterioration of the LED array.
To prevent the foregoing problems, in the conventional image reading apparatus, light source lighting control as will be explained hereinbelow is performed.
First, FIG. 1 is a schematic constitutional diagram of an image input unit. In this diagram, a plurality of photo sensors 1 are arranged in an array form in such a manner that the optical axis of each photosensor 1 coincides with the optical axis of a lens array 2 of the refractive index distribution type. An LED array 3 irradiates the lights to a reading position P of an original 4 with light. The reflected lights passes through the lens array 2 and is focused onto the photosensor 1.
FIG. 2 is a schematic diagram showing an example of an arrangement of the photosensors 1 and LED array 3. As shown in the diagram, sensor cells S.sub.1 to S.sub.40 of the photosensor 1 are divided into sensor blocks SB.sub.1 to SB.sub.4, and the LED array 3 is divided into light source blocks 5 to 8.
The photosensor 1 and LED array 3 divided into a plurality of blocks in this manner are controlled at the timings shown in the timing chart of FIG. 3.
As shown in the sense timing in FIG. 3, the sensor blocks SB.sub.1 to SB.sub.4 of the photosensor 1 are sequentially driven on a block unit basis. The light source blocks 5 to 8 are also sequentially lit synchronously with the driving of the sensor blocks (the high level denotes the light-on and the low level indicates the light-off). As shown in the diagram, the light source blocks 5 to 8 are lit at the timings in such a manner that the half period of light-on times overlap each other.
Owing to such a light-on control, the light is incident on the photosensor 1 for a predetermined period of time before it performs the reading operation, so that the limitation of the reading speed due to the light response time can be eliminated. Further, since the LED array 3 is divisionally sequentially lit the increase in temperature is suppressed and the reduction of the light amount can be prevented.
However, in the image reading apparatus such as facsimile apparatus or the like having the image transmitting function, if an amount of information of the original is large, the conversion processing time in the modem increases and after one reading scanning time of the photosensor 1, it also takes a long time to transfer the image data.
To perform the lighting control at the timings shown in FIG. 3 in such a case, the period of time until the transfer is completed is ordinarily set to the waiting time until the reading operation is started. However, in the conventional image reading apparatus, the photosensor 1 is sometimes left in the dark for a long time. Consequently, there is such a problem that when the next main scanning operation is started, the photo current rises with a delay time and the output decreases.